This invention relates to remote load positioning, and more particularly to remote placement of a plug or similar article on a target within a manually inaccesible area of a nuclear power plant, such as primary system piping.
In many pressurized water reactor piping designs, provision is made for auxiliary shutdown cooling loops to remove residual heat from the primary coolant once the reactor has been shutdown, depressurized, and the primary circulating pumps deactivated. Typically, such loops will draw coolant from a hot leg connecting a steam generator to the reactor vessel, pass the coolant through a heat exchanger, and return the lower temperature coolant to the reactor by means of one of the primary system cold legs. These loops are not used during normal reactor operation and are typically isolated from the system by valves. Past practice has allowed for pipe maintenance on shutdown cooling loops and their isolation valves by providing valves on the reactor hot and cold legs, and thus the shutdown coolant loops; or by physically locating shutdown coolant loop isolation valves above the level of reactor coolant needed for shutdown cooling, thus permitting valve maintenance or replacement without the loss of coolant necessary to maintain the reactor core at specified shutdown temperatures.
For piping systems with none of the above provisions, a means is needed for isolating each of the shutdown coolant loops independently of other loops which provide the cooling function while the defective loop is repaired. One solution is to remove the reactor vessel head and the upper vessel internals, and to remove all the fuel assemblies comprising the reactor core to temporary fuel storage. In this case, the shutdown coolant loops are not needed and the loops may be drained of coolant to permit the necessary maintenance. This method is unacceptable to most reactor operators because of the time and cost to remove and replace the entire core which adds to the reactor's unavailability to produce useful energy.
A second solution is to remove the reactor vessel head and the upper vessel internals. This condition allows access to the reactor vessel hot leg nozzles while the fuel assemblies are still in place in the reactor core. Remote tooling may then be utilized to sequentially plug each of the hot leg nozzles associated with shutdown coolant loops requiring maintenance. Such an operation would require the reactor plant to be placed in the refueling mode with the refueling pool flooded even if a refueling outage were not scheduled.
A third solution is to install a plug in the branching nozzle of the hot leg which is used to draw coolant into the shutdown coolant loop. In this case, the reactor needs only to be placed in the shutdown mode, and the vessel head and internals may remain in place. The plug and tooling are entered into the drained steam generator primary head, assembled, and the plug installed in the branching nozzle by an operator manipulating the tooling described in this disclosure.